1. Field of the Invention
The invention relates generally to the field of digital signal communications and to receive and transmit chain calibration. More particularly, the invention relates to calibrating a wideband or spread spectrum communications system using narrowband signals.
2. Description of the Related Art
Radio communications capacity can be greatly increased using directional, rather than omnidirectional radio transmission. One way to transmit directional signals and directionally receive signals is by using beam forming and nulling through an array of antennas. The precision of the beam forming and nulling through an antenna array, can be improved if the transmit and receive chains are both calibrated. Calibration can be applied to the chain from the digital interface at baseband to the field radiated from or received at each antenna element. One way of making the calibration is to have a transponder separated from the antenna array listen to the output of the antenna array on a base station downlink frequency. The transponder receives a downlink calibration signal from the base station and then re-transmits it on an uplink frequency. By selecting appropriate signals to transmit and appropriate signals to receive, the base station can apply signal processing to estimate compensations in phase and amplitude to calibrate its transmit and receive chains.
A remote transponder calibration system is shown, for example, in U.S. Pat. No. 5,546,090 to Roy, III et al. That patent describes a narrowband FDD (frequency division duplex) system. In an FDD system, unused time and frequency slots typically occur on occasion and these can be used to send and receive a narrowband calibration signal. In a typical spread spectrum system, however, there are no unused time and frequency slots to use for calibration. A spread spectrum system, for example a CDMA (code division multiple access) system, as opposed to FDMA (frequency division multiple access) and TDMA (time division multiple access) systems, has multiple users using the same radio channel at the same time. If the transponder is designed to receive and transmit the signal using the same spread spectrum channel that is used for traffic, then the additional energy added to the channel by calibration will reduce system capacity. A typical transponder will receive all of the downlink traffic including the calibration signal, shift the frequency, amplify it and send all of the traffic back to the base station. This results in a very large amount of energy being sent by the transponder on the uplink and may effectively overpower all other traffic. As a result, calibration will affect both the downlink and uplink channel capacity.
The interference can be reduced by turning off all normal downlink signals when calibration is being performed and sending a special low interference calibration downlink signal. However, in addition to interrupting the downlink traffic, this approach will still cause interference on the uplink. The interference can also be reduced if the transponder is constructed to despread the calibration signal to isolate it from all other traffic and then send back only a spreaded, frequency shifted version of that despread calibration signal. This approach results in a more expensive transponder and renders the calibration of the transit chain more difficult.